1. Field of the Invention
The present invention relates to a method for aligning a substrate in a lithographic apparatus and to a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithography, such as for the manufacture of integrated circuits or other semiconductor structures, it is common that a plurality of layers are applied onto the substrate. This implies that a substrate is to be processed a plurality of times. Each time a pattern is to be created, the substrate may be provided with a layer of a photosensitive resist, followed by irradiating by the lithographic apparatus a pattern onto the substrate, thereby selectively irradiating a desired portion of the resist applied to the surface of the substrate. After a further processing of the substrate has taken place, which may include any suitable processing step, such as etching, ion implantation, diffusing, or any other operation which may be known to the skilled person per se, a following irradiation is to take place. Due to an ever increasing complexity of semiconductor structures or other devices to be produced with the aid of the lithographic apparatus, requirements on a complexity of the pattern, e.g. increasing a resolution thereof, decreasing a size of elements of the pattern, etc, and requirements on alignment of the substrate increase. In this context, the term alignment is to be understood as an alignment of a substrate on which a previous pattern has been applied, to the patterned beam of the lithographic apparatus for applying a following pattern onto the substrate. This may be easy understood by realizing that the smaller the structures of the pattern to be applied onto the substrate, the more critical a matching between a position of the following pattern with respect to the pattern already applied onto the substrate, becomes.
In a current lithographic apparatus a plurality of means are used to be able to provide an adequate alignment. Most of these are position measurement devices to measure a position of a substrate table holding the substrate, position measurement devices to measure a position of a support to support a patterning device which provides the pattern to be irradiated. Further, a measurement system may be provided to optically measure a relationship between the support and/or patterning device and the substrate table. Thereto, a measurement device may be provided on the substrate table (e.g., comprising a lightsource and a detector). Thereby, a relationship between the substrate table and the support or patterning device may be determined with the aid of a measurement beam which is projected from the support or patterning device onto the substrate table, and/or vice versa, via a projection system. An example thereof are the so-called TIS (Transmission Image Sensors) which may be provided on the substrate table, and measures its position with respect to a projection beam that is formed by irradiating a particular pattern provided on the patterning device (a marker). Any of these known measurement systems are prone to errors which may be caused by a plurality of effects. At first, many of the measurements determine a position of the patterning device towards the substrate table instead of to the substrate itself. Similarly, commonly a position of the support to hold the patterning device is determined instead of the position of the patterning device itself. Furthermore, the substrate table and/or the support may comprise a large range of movement which implies that sensing systems being able to operate over such a large range of movement are to be applied. Therefore, optical beams or other means may have to travel long measurement paths which may cause errors due to atmospheric gas turbulences, temperature variations, etc. Furthermore, these measurement systems do not measure a relative position of patterning device and substrate with respect to each other, however measure, e.g., the substrate table with respect to a reference and/or the support or patterning device with respect to another or the same reference. Thereby, a long measurement path comes into existence, e.g. ranging from the substrate via the substrate table via a measurement beam to, e.g., a measurement device (such as an interferometer, encoder, etc.), via a reference structure (such as a metrology frame), via a further measurement device (such as a interferometer, encoder etc.), then again via a measurement beam to the support and finally from the support to the patterning device. Thus, such a measurement path includes a plurality of positioning relationships between the various elements. In each of these elements/or relations therebetween, errors may be introduced due to temperature fluctuations (causing thermal expansion of one or more elements, due to position inaccuracies of an element with respect to another one (e.g., a position inaccuracy of the substrate with respect to the substrate table), etc. Furthermore, another source of error is in an initial positioning of a layer onto the substrate. When the layer is processed in a first processing cycle where, e.g., a first pattern is applied onto the substrate, a position of that pattern with respect to a reference on the substrate is to be determined. Such a reference may, e.g., be a border of the substrate or comprise reference markers on the substrate itself. It is possible that these reference markers are included in the pattern to be applied first onto the substrate, thereby aiding in a following positioning of the substrate to apply a further pattern.
Despite the many different measurement systems and techniques used, a highly accurate alignment of the substrate is still considered troublesome and thereby limiting a performance of the lithographic apparatus, e.g. in terms of a match between subsequently apply layers, and therefore in terms of minimum dimensions of the structures to be achievable in such layers.